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Multiplication in Rijndael's field; usual polynomial multiplication followed by reduction
by the irreducible polynomial. The irreducible used by Rijndael's field is the polynomial
x^8 + x^4 + x^3 + x + 1, which we write by giving it's exponents in SBV.
See: http://en.wikipedia.org/wiki/Finite_field_arithmetic#Rijndael.27s_finite_field.
Note that the irreducible itself is not in GF28! It has a degree of 8.

NB. You can use the showPoly function to print polynomials nicely, as a mathematician would write.